EP3186866A1

EP3186866A1 - Explosion protection circuit with impedance matching

Info

Publication number: EP3186866A1
Application number: EP15753028.8A
Authority: EP
Inventors: Achim Wiest; Andreas Berger; Oliver Brumberg
Original assignee: Endress and Hauser Flowtec AG
Current assignee: Endress and Hauser Flowtec AG
Priority date: 2014-08-29
Filing date: 2015-08-19
Publication date: 2017-07-05
Also published as: DE102014112452B4; US10461526B2; CN107079202A; US20170256937A1; WO2016030251A1; DE102014112452A1

Abstract

A device for monitoring at least one physical or chemical process variable, comprising at least one sensor unit (2) and an electronics unit (4) for signal detection, signal evaluation, and/or signal power supply, wherein the sensor unit (2) is driven using alternating current and/or the communication between the electronics unit (4) and the sensor unit (2) is carried out using alternating current and/or alternating voltage, and comprising an explosion protection circuit (3) with intrinsic safety, which comprises a safety barrier having at least one unit in each case for current limiting and/or voltage limiting, wherein a unit is provided inside of the explosion protection circuit (3) for impedance matching, which unit comprises at least one transformer (12) for impedance matching.

Description

Explosion protection circuit with impedance matching

The invention relates to a device for monitoring at least one

physical or chemical process variable comprising at least one

Sensor unit and an electronic unit for signal acquisition, evaluation and / or - supply, the device is suitable for use in hazardous areas.

To operate a field device in an explosive atmosphere

different measures necessary. These have the aim of preventing the formation of a spark, which could possibly cause an explosion, or to prevent a spark generated inside a closed space from having an effect on the environment. In this respect, different protection areas are distinguished, and accordingly there are different standards for different ones

Environmental conditions and applications. The standard DIN EN60079-1 1 defines the necessary safety requirements for a device protection by intrinsic safety, known under the abbreviation Ex-i. This is a type of protection based on the limitation of electrical energy within equipment and / or

Connecting cables, which are exposed to a potentially explosive atmosphere, to a level below that at which ignition can be caused either by sparking or heating. In addition, different protection levels can still be distinguished within this protection class, Ex-ia, Ex-ib and Ex-ic. These additions define the present reliability of the individual components and connecting cables.

For explosion protection in accordance with class Ex-i, it must be ensured in particular that the electrical quantities of current, voltage and power in a device are always below a specified limit value at all times. These limits are then chosen so that in case of error, z. B. by a short circuit, the maximum amount of heat is insufficient to generate a spark. For this purpose, the current is often due to resistors, the voltage through diodes, esp. Zener diodes, and the power through a suitable combination of Ström- and

voltage-limiting components kept below the specified limits. Such a protective circuit can be described, for example, under the name MCR- PLUGTRAB PT can be purchased from the company Phoenix Contact as a pluggable device. With regard to field devices, several examples are still known in which the requirements for explosion protection by special

Circuit arrangements are met, for example in EP0882955A1,

DE29809853U1 or WO2004 / 098014A1. The latter document describes in particular for the units for current and voltage limiting arrangements of

Vorwiderständen while DE29809853U1 provides for the use of Zener diodes. However, the use of diodes has the disadvantage that the applications are limited to a certain temperature range.

Furthermore, the above examples of explosion protection circuits have no impedance matching. This applies in particular to field devices in which the communication between the electronics unit and the sensor element takes place by means of alternating current or alternating voltage. Typical frequencies are in the range of 10kHz to 50MHz. The problem is that the load impedance of the sensor element is often not matched to the line or electronics unit, resulting in standing waves on the line. This will not transfer the full power. By way of example, such field devices are cited in which the sensor element is acted upon by at least one piezoelectric element. This applies among other ultrasonic flowmeters or vibronic level gauges, as distributed by the applicant in great diversity, in the first case under the

Designation Prosonic DDU10, Prosonic Proline P, or Prosonic Flow and in the second case under the name of Solifant and Liquifant. A typical method for impedance matching, especially to a

To ensure piezoelectric element consists in the use of coils or

Transformers. The respective components must be adapted in terms of their characteristics to the line, to the electronic unit, which in this case usually has an amplifier, and to the piezoelectric element itself.

The present invention has for its object to provide a field device with an explosion protection circuit, which allows the operation of the field device in an explosive atmosphere while transmitting the signals with minimal power loss. This object is achieved according to the invention by a device for monitoring at least one physical or chemical process variable

at least one sensor unit and an electronic unit for signal acquisition, evaluation and / or supply, wherein the sensor unit is operated with alternating current and / or the communication between the sensor unit and the electronic unit with AC and / or AC voltage, and with a

Intrinsically safe explosion protection circuit comprising a safety barrier each having at least one unit for limiting current and / or voltage, wherein a unit for impedance matching is provided within the explosion protection circuit, which unit comprises at least one transformer for impedance matching. The impedance matching guarantees a lossless as possible

Transmission of the signals and correspondingly that more energy for the sensor unit is available, as a universal adjustment over a large interval between the impedances of the sensor unit and the electronic unit is possible.

It is advantageous if the unit for impedance matching is designed so that the sensor unit and the electronics unit are galvanically separated from each other. An appropriate design of the transformer also ensures a

Zone separation. This is particularly advantageous for media-contacting sensor units with a metal housing and a cathode protection on a pipeline carrying the medium. The cathode protection means in such an application that the potential of the pipeline to the ground connection by means of a voltage source is kept at a certain value, in particular for example U = 5V, in order to prevent corrosion of the pipeline. By using an explosion protection circuit with a galvanic isolation between

Sensor unit and electronic unit is enabled in this case, an independent operation of the circuits. This is often not the case with conventional explosion protection circuits. For this it is advantageous if the transformer is designed in such a way, in particular by ensuring sufficiently large distances and suitable choice of material such as paints and insulating films, that it ensures the galvanic separation. In a preferred embodiment, the current limiting unit comprises at least one resistor. Likewise, it is beneficial if the unit for

Voltage limitation comprises at least one coil. Such a choice of the voltage-limiting components allows the use of the device according to the invention in a wide temperature range, in particular is the possible

Temperature range greater than when using diodes.

It is also advantageous if in each case the same components in the units for current limitation, voltage limiting and impedance matching are designed such that they have a dual function. Thus, for example, the voltage limiting unit and the impedance matching unit may be combined via the transformer. Furthermore, the DC resistances of the windings of the coils can also act as a flow limiting. In another preferred embodiment, the sensor unit has at least one piezoelectric element. This is particularly true when using the device according to the invention for ultrasonic flowmeters or for vibronic

Level gauges the case. The at least one process variable is then

for example, given by the flow of a medium through a pipe, the level of a medium in a container or by the density or viscosity of the respective medium.

It is also advantageous if the explosion protection circuit between the

Sensor unit and the electronic unit is fixed. Alternatively, the

Explosion protection circuit may also be arranged in a separate plug-in adapter, which plug-in adapter is mounted retrofittable between the sensor unit and electronic unit. In this case, already existing devices can be retrofitted with an explosion protection circuit and / or impedance matching. In a particularly preferred embodiment, the number of parallel branches determines the reliability, in particular there is at least a simple reliability when the explosion protection circuit has at least two parallel branches, and there is at least a double failure safety at least three parallel branches. This corresponds to the standard Ex-ib. In a double Resilience, which is achieved in the case of three parallel branches, even the standard Ex-ia is met.

It is advantageous if at least one component, which is provided within the units for current and / or voltage limiting, is executed several times. This concerns in particular the case with two parallel branches within the

Explosion protection circuit, since the two branches are then designed equivalent. In the case of a simple failure safety, for example, two current-limiting resistors and two voltage-limiting coils would be designed to be redundant, while in the case of double failure safety three coils and three resistors would be redundant.

Furthermore, it is advantageous if at least one of the components is designed to be fail-safe. This can be achieved for example by coils with a wire thickness of 0> 0.05 mm and a minimum breakdown voltage according to IEC of grade 2, by resistors, which are designed as wire wound resistors and which a

have sufficient temperature resistance, and / or by conductor tracks with a corresponding cross section, in particular height and width. In another particularly preferred embodiment, the

Explosion protection circuit on a switching function. With this switching function, at least one unit to ensure the intrinsic safety, in particular the unit for current limitation, bridged, in particular short-circuited. Thus, in the non-hazardous area, the transformer can be used without a current limitation, while in the Ex area a variant with current limitation and zone separation is provided.

It is particularly advantageous if the switching function can only be changed with a special tool, in particular with a key switch, or if it is located in an area of the device which is accessible only by means of a special tool. It is advantageous if the areas of the device, which are possibly exposed to an explosive atmosphere, are castable and / or potted. It is also advantageous if between the explosion protection circuit and

at least one other component of the device consists of at least one compound, and if each compound is solvable only by tools, or is insoluble.

The invention will be described in greater detail below on the basis of several exemplary embodiments by means of the enclosed figures:

It shows:

Fig. 1 is a block diagram of a field device according to the prior art

Fig. 2 is a circuit diagram of an explosion protection circuit according to the invention with simple reliability

Fig. 3 is a circuit diagram of an explosion protection circuit according to the invention with a switching function according to the invention.

Fig. 4 shows a circuit diagram of an explosion protection circuit according to the invention with double failure safety Fig. 1 shows a simplified block diagram of a device 1 according to the prior

Technology, such as a field device. The field device can be given for example by a working according to the ultrasonic flowmeter. Such field devices are manufactured by the applicant in great diversity and distributed for example under the name Prosonic DDU10 or Prosonic Proline P. However, it goes without saying that other types of field devices are also covered by the invention. The sensor unit 2 and the electronics unit 4, between which an explosion protection circuit 3 according to the invention is integrated, are indicated. This can be either fixed, or in a separate plug-in adapter which is detachably mounted between the electronic unit 3 and the sensor unit 2. Various variants are possible for the explosion protection circuit, of which three different examples are to be shown in detail below. It goes without saying that many more arrangements are conceivable, which also fall under the invention.

Fig. 2 shows a circuit diagram of an explosion protection circuit 3 'according to the invention with simple reliability, which between the electronic unit 4 and the

Sensor unit 2 is arranged. The sensor unit 2 comprises a piezoelectric element 5, which is located within a metallic shield and to which the impedance is adapted. The signal-carrying lines are given by Triaxialkabel 6, of which a conductor is in each case connected to the metallic housing 6b. For impedance matching, a transformer 12 with at least three windings is used. As a rule, transformers are designed with two windings. The third winding is used here accordingly not the impedance matching, but the explosion protection. Thus, transformer 12 is shown in FIG

Embodiment a dual function too. In addition to the at least one additional winding, it is advantageous if the winding sense on the side with the

Piezo element 5 opposite to that on the side to the electronics unit 4 out. In the event of a fault, this reduces the stored energy.

The following descriptions refer exclusively to the

Explosion protection circuit 3 '. The two circuit branches on the left side each have a coil 7, 7a, each with a line resistor 9, 9a, and one series-connected resistor for current limiting 8, 8a. This redundant structure ensures easy failure protection of the explosion protection circuit 3 '. In a third circuit branch on the side with the piezoelectric element 5, there is another coil 7b and the associated line resistance 8b. By suitable choice of the inductances and mechanical design of the coils 7, 7a, 7b, both the power can be suitably transformed, as well as a zone separation can be ensured. The resistors 8,8a turn serve to limit the current. If one chooses to low-resistance, the current is not sufficiently limited and the circuit ensures only an impedance matching. If the device is operated in a non-explosive environment, such a choice of resistors 8, 8a is advantageous since more energy is transmitted to the piezoelectric element 5 can be. An explosion protection effect can only be achieved if enough high-resistance resistors 8,8a are used. So it depends on the choice of the appropriate components and the desired application. In the following, the explosion protection effect of the circuit shown in Fig. 2 will be shown with reference to a concrete example. For this purpose, specific values are selected for the individual components and two energy considerations are carried out.

The inductive energy in a coil 7, 7a, 7b is given by

1

'ind -LI ²

2

Considering a standard safety factor of 1.5 for the current, a coil 7b with Ι_ = 1500μΗ and E _{n (i} = 20 iJ (maximum allowed energy in the coil: considering a safety factor of 2) results in a maximum current of

2 X 20μ |

1500μΗ

I = * = 0.1634

1.5

and accordingly for the resistance at U = 10V (DC in case of error)

U 10V

R = - = = 61.4Ω

/ 0.1634

Depending on the choice of the series resistor 10 of the output stage 4a within the

Electronics unit 4 are therefore additional resistors 8,8a necessary to achieve a sufficient current limit. A typical value for the series resistor of the output stage 4a is Thus, the parallel circuit R _{s of} the two resistors 8, 8a would have to be selected to be at least high enough that R _s = 61.4μ-50Ω = 11.4Ω. Furthermore, a second resistor 10a is provided, the internal resistance of the input stage 4b. It is necessary for a reflection-free transmission of the

Response signal and typically will be for him the same value as that for the

Series resistor 10 selected. In an adaptation of the AC voltage in the operation of the device, the sum of the impedances 7,7a, 7b and resistors 8,8a should ideally equal to the value of

while Rio again correspond to the characteristic impedance of the triaxial cable 6. Also at the operating frequency should be the phase of the protection circuit with connected piezo element 5 cp = 0 °. The resistors 8b, 9 and 9a are the DC resistances of the windings.

A slight modification of the circuit of Fig. 2 is the insertion of the compound 1 1. In this case results due to the parallel circuit R _s = 22.8ü. Thus, the additional connection 1 1 provides for an increase of the resistance R _s and thus for a reduction of the current. On the other hand, when the connection 1 1 is omitted, less voltage drops across the resistors 8, 8a, so that more energy is available to the piezoelectric element 5.

The same applies to the so-called impact test for a mechanical impact with 7J at the piezo element 5. The energy should not exceed a value of 50 μΐ according to standard without inductance.

For the capacitive energy applies

Ekap ^~ 2 ^■

For a piezo element with C = 600pF, a capacitive energy of E _kap = 20μ] and considering a safety factor of 2.5 for the energy, this results in the voltage at the piezoelectric elements

The maximum stress would therefore have to lie below U _piezo = 2S8V in the case of a mechanical impact of 7J. By using a coil 7b with

Ι_ = 1500μΗ on the side with the piezoelectric element and the other two coils 8,8a, this voltage is not reached.

A second example of an explosion protection circuit 3 "is shown in Fig. 3. Fig. 3 differs from Fig. 2 only by the addition of a switch 13. This allows the bridging of the connected in series with the coils 7,7a

Resistors 8,8a, so that the device either with or without

Explosion protection can be operated. This makes the decision superfluous between high and low resistors 8,8a because they can be bypassed at any time by means of the switching function.

A final example is shown in FIG. This is a

Explosion protection circuit 3 "with double failure safety

present example by a third circuit branch 14 on the

Piezo element 5 opposite side reached. In each of the circuit branches, a resistor 8,8a, 8c are connected in series with each of a coil 7, 7a, 7c, and a compound of the three circuit branches 1 1 a added.

An alternative measure is to ensure a sufficiently large distance between the circuit containing the piezoelectric element 5 and the circuit containing the three switching branches. This allows higher voltages at the

Piezoelectric element with low risk of a rollover during operation can be used.

It goes without saying that the essential functional units of the three examples shown can be freely combined with one another. This concerns the choice of the type of reliability, the integration of additional switches such as the switching function 13 in Fig. 3, or the additional connection 1 1, 1 1a. With regard to the individual components, however, the resonant circuits must then be matched to the respective application.

In a fixed arrangement of the explosion protection circuit, this may be arranged, for example, together with the piezoelectric element 5-containing circuit in a molded housing. But other arrangements are of course conceivable.

LIST OF REFERENCE NUMBERS

1 field device

2 sensor unit

3 explosion protection circuit

4 electronic unit

4a, 4b input stage and output stage of the electronics unit

5 piezo element

6 triaxial cables

6a Connection of a conductor of the triaxial cable with the metallic one

casing

7, 7a, 7b coils

8.8a-f resistors

9, 9a, 9b line resistances of the coils

10.10a series input resistors and output stage

1 1, 1 1 a compound

12 transformer

13 switching function or switch

14 fourth switching branch

Claims

Patent claims

Device for monitoring at least one physical or chemical process variable, comprising at least one sensor unit (2) and one

Electronic unit (4) for signal acquisition, evaluation and/or supply, the sensor unit (2) being operated with alternating current and/or the

Communication between the electronic unit (4) and the sensor unit

(2) takes place with alternating current and/or alternating voltage,

and with an explosion protection circuit (3) with intrinsic safety, which has a safety barrier with at least one unit for power and/or

Voltage limitation includes,

characterized by,

that within the explosion protection circuit

(3) a unit for

Impedance matching is provided, which unit for impedance matching comprises at least one transformer (12).

Device according to claim 1,

characterized by,

that the unit for impedance matching is designed in such a way that

Sensor unit (2) and the electronics unit

(4) are galvanically isolated from each other.

Device according to claim 1 or 2,

characterized by,

that the transformer (12) is designed in this way, in particular by the

Ensuring sufficiently large distances and suitable choice of materials such as paints and insulation foils ensures galvanic isolation.

Device according to at least one of the preceding claims,

characterized by,

that the current limiting unit comprises at least one resistor (8, 8a, 8c).

5. Device according to at least one of the preceding claims,

characterized by,

that the voltage limiting unit comprises at least one coil (7,7a,7b,7c).

6. Device according to at least one of the preceding claims,

characterized by,

that the same components are used in the current limiting units,

Voltage limitation and impedance matching are designed in such a way that they have a dual function.

7. Device according to at least one of the preceding claims,

characterized by,

that the sensor unit has at least one piezo element (5).

8. Device according to at least one of the preceding claims,

characterized by,

that the explosion protection circuit (3) is permanently arranged between the sensor unit (2) and the electronic unit (4).

9. Device according to at least one of claims 1 to 7,

characterized by,

that the explosion protection circuit (3) is arranged in a separate plug adapter, which plug adapter can be retrofitted between the sensor unit (2) and the electronic unit (4).

10. Device according to at least one of the preceding claims,

characterized by,

that number of parallel branches within the explosion protection circuit (3) ensures reliability, in particular that at least one simple one

Reliability exists if the explosion protection circuit (3) has at least two parallel branches, and there is at least double reliability with at least three parallel branches.

1 1. Device according to at least one of the preceding claims, characterized in that

that at least one component, which is provided within the units for current and/or voltage limitation, is designed multiple times.

12. Device according to at least one of the preceding claims,

characterized by,

that at least one of the components is designed to be fail-safe.

13. Device according to at least one of the preceding claims,

characterized by,

that the explosion protection circuit (3) has a switching function (13), and that with the switching function (13) at least one unit to ensure intrinsic safety, in particular the unit for current limitation, can be bridged, in particular short-circuited.

14. Device according to claim 13,

characterized by,

that the switching function (13) can only be changed with special tools, in particular with a key switch, or if it is in a

Area of the device which is only accessible using special tools.

15. Device according to at least one of the preceding claims,

characterized by,

that the areas of the device (1), which may be one

are exposed to explosive atmospheres, are castable, and/or are cast.

16. Device according to at least one of the preceding claims,

characterized by,

that there is at least one connection between the explosion protection circuit (3) and at least one further component of the device (1), and that every connection can only be detached using tools, or is inseparable.